Jack J. Hsia

Reflection properties of pressed polytetrafluoroethylene powder

The reflection properties of pressed polytetrafluoroethylene powder have been under investigation by the Radiometric Physics Division at the National Bureau of Standards for the past five years. This material has a great potential use, both as a standard of diffuse reflectance and as a coating for integrating spheres for applications in reflectance spectrophotometry and other signal-averaging devices. It possesses certain physical and optical properties that make it ideal for use in these applications. Techniques are given for preparing reflection standards and coating integrating spheres with the pressed powder. The effects of powder density and thickness on its reflectance are reported, and observations of possible problems with fluorescence that are due to the presence of contaminants in the powder are discussed. The absolute reflectance (6°/hemispherical reflectance factor relative to a perfect diffuser) is reported for the spectral range of 200–2500 nm. The directional/hemispherical reflectance factor relative to 6°/hemispherical reflectance is given for several wavelengths in the ultraviolet and visible spectrum and for angles of incidence between 5 and 75°. The bidirectional reflectance factor is reported for 300, 600, and 1500 nm at angles of incidence of −10, −30, −50, and −70° and at viewing angles at 10° intervals from −80 to +80°.

Laboratory intercomparison study of pressed polytetrafluoroethylene powder reflectance standards.

The object of this paper is to present results of several experiments relating to the preparation and use of pressed polytetrafluoroethylene (PTFE) powder as a diffuse reflectance standard for the 200–2500-nm spectral range. These experiments include two round-robin intercomparisons involving nine laboratories. These round-robin experiments provided data on the variability of the reflectance of pressed PTFE reflectance standards prepared in different laboratories. Results of these measurements provided insight into the problems associated with the PTFE standards and helped to establish what practices needed to be standardized to improve interlaboratory agreement for diffuse reflectance measurements.

NBS 45°/Normal Reflectometer for Absolute Reflectance Factors

A 45°/normal reflectometer has been constructed and tested for calibrating the absolute reflectance factor of diffuse samples over the 380-770 nm spectral range using polarized radiation. The measurement equations have been derived for the method used. The method using a step-down technique and view factor calls for the measurements of the ratio of two fluxes and, in addition, some linear dimensions. The uniformity of the receiver system is achieved by means of a double-sphere signal averager. Uncertainties of the absolute-reflectance-factor measurements obtained with this system are estimated to be ±0.3% of the measured value. For all the samples that have been tested, the 45°/normal reflectance factor was found to be higher than the 6°/hemispherical reflectance factor. The higher reflectance values for 45°/normal geometry were confirmed by additional gonioreflectometer measurements.

The 1990 NIST Scales of Thermal Radiometry

Following an absolute NIST measurement of the freezing temperature of gold and the adoption of the International Temperature Scale of 1990 (ITS-90), NIST has adopted new measurement scales for the calibration services based on thermal radiometry. In this paper, the new scales are defined and compared to the ITS-90, and the effects of the scale changes on NIST measurement services in optical pyrometry, radiometry, and photometry are assessed quantitatively. The changes in reported calibration values are within quoted uncertainties, and have resulted in small improvements in accuracy and better consistency with other radiometric scales.

NBS specular reflectometer–spectrophotometer

A specular reflectometer has been constructed and tested for calibrating the reflectance of mirror standards over the 250-2500-nm spectral range. This instrument is a measurement accessory to a reference spectrophotometer, which is also used for diffuse hemispherical spectral reflectance and 45 degrees /0 degrees spectral reflectance. The specular reflectometer is designed to measure mirror reflectances at angles of incidence between 5 and 80 degrees using both vertically and horizontally polarized radiation. Absolute reflectance measurements are obtained by an optical system, which provides for direct measurement of the incident beam and for the sample mirror reflectance using the same beam. This is accomplished by means of a beam tracking system through which the beam is directed into a signal averaging sphere. The sphere rotates with the beam tracking optics, and the stationary detector views the interior of the sphere. Control of the beam tracking optical system is accomplished by a computer-controlled stepping-motor-driven precision turntable. Uncertainties of the reflectance measurements obtained with this system are estimated to be +/-0.2% of the measured value.

NBS reference hazemeter: its development and testing

A research hazemeter has been developed by the Radiometric Physics Division of the National Bureau of Standards for the quantitative analysis of light scattering by optical materials. The instrument is designed to measure percentage haze as set forth in the prescribed conditions recommended in ASTM Test Method D 1003. The hazemeter has several special features which make it possible to vary the geometrical conditions of the collimated light beam and the integrating sphere detector system for detailed studies of light scattering. Light scattering associated with the instrument source and collimating optics has been reduced to less than 0.1% of the total light beam by using a diffraction limited off-axis parabolic mirror to collimate the hazemeter light beam. The source optics provide a collimated light beam of approximately 6800 K. The detector is a silicon cell photodiode with a V(lambda) window. This combination gives the system a response corresponding to the visual luminosity function. The detector nonlinearity and instrumental errors associated with the accuracy of the integrating sphere geometry have been analyzed. The over-all uncertainty in percentage haze has been reduced to +/-0.2%, and light scattering of less than 0.5% can readily be measured.

Rayleigh scattering limits for low-level bidirectional reflectance distribution function measurements

The objective is to estimate the Rayleigh limit in bidirectional reflectance distribution function (BRDF) measurements caused by air in the laboratory, the wavelength, and the path length of light in the receiver field of view. Moreover, we intend to show the trend for the reduction of this limit by introducing a medium with small refractive index and by using a longer wavelength. Although the BRDF typically describes the angular distribution of scattered light from surfaces, the expression describing the equivalent BRDF caused by the optical scattering from gas molecules in the optical path is derived through the use of the Rayleigh scattering theory. The instrumentation is described, and the experimental results of the equivalent BRDF caused by gas scattering from molecules in clear air, nitrogen, and helium gases are reported. These results confirm the trends of the prediction.

Instrumentation at the National Institue of Standards and Technology for bidirectional reflectance distribution function (BRDF) measurements

A description is given of the goniometric optical scatter instrumentation (GOSI) which has been developed at NIST for measuring the bidirectional reflectance distribution function (BRDF) from surfaces. The source, goniometer, and receiver are described. The systematic optical and electronic noise of the system is reported. The systematic electronic noise and background scatter have been reduced to allow a detection limit of 1 X 10-9 sr-1. This value is the Rayleigh scattering level from the air molecules in the room with a field of view of 7.2 cm at the sample position for 633 nm incident laser light.

NBS reference retroreflectometer

A long-range retroreflectance instrument has been built in the photometric range of the Radiometric Physics Division of the NBS. It is designed to measure photometric properties of retroreflectors for different geometries. It satisfies many needs of the measurement community, and it is planned to use this instrument as the basis for a Measurement Assurance Program (MAP) and for Standard Reference Materials (SRM). This paper describes the design and testing of the instrument. Some estimated uncertainties for typical samples are given.

UV bidirectional reflectance distribution function measurements for diffusers

Bidirectional reflectance distribution function (BRDF) is an important quantity to describe the scattering condition from a diffuse surface. The current presentation describes the instrumentation, instrument characterization and diffuser calibration in the ultraviolet (UV) spectral region.